The present invention relates to photosensitive optical waveguides. In particular, the present invention relates to optical fibers having selected and discrete longitudinal regions of high-photosensitivity.
Optical fibers and optical fiber devices are widely used in signal transmission and handling applications. Optical fiber-based devices are vital components in today's expanding high-volume optical communications infrastructure. Many of these devices rely on fiber Bragg gratings (FBG's) to perform light manipulation. An FBG is an optical fiber with periodic, aperiodic or pseudo-periodic variations of the refractive index along its length in the light-guiding region of the waveguide. The ability to produce these refractive index perturbations in a fiber is necessary to manufacture FBG's and, hence, a number of optical components, such as optical sensors, wavelength-selective filters, and dispersion compensators.
Gratings are written in optical fiber usually via the phenomenon of photosensitivity. Photosensitivity is defined as the effect whereby the refractive index of the glass is changed by actinic radiation-induced alterations of the glass structure. The term “actinic radiation” includes visible light, UV, IR radiation and other forms of radiation that induce refractive index changes in the glass. A given glass is considered to be more photosensitive than another when a larger refractive index change is induced in it with the same delivered radiation dose.
The level of photosensitivity of a glass determines how large an index change can be induced in it and therefore places limits on grating devices that can be fabricated practically. Photosensitivity also affects the speed that a desired refractive index change can be induced in the glass with a given radiation intensity. By increasing the photosensitivity of a glass, one can induce larger index perturbations in it at a faster rate.
The intrinsic photosensitivity of silica-based glasses, the main component of high-quality optical fibers, is not very high. Typically index changes of only approximately 10−5 are possible using standard germanium doped fiber.
However, it has been observed that by loading the glass with molecular hydrogen before irradiating it with actinic radiation, one may increase significantly the photosensitivity of the glass. Exposing Ge-doped silica optical fibers to hydrogen or deuterium atmospheres at certain temperatures and pressures photosensitizes the fibers. Index changes as large as 10−2 have been demonstrated in hydrogenated silica optical fibers.
Prior references have emphasized upper limits on the temperature for such hydrogen loading. For example, U.S. Pat. Nos. 5,235,659 and 5,287,427 discuss a method for exposing least a portion of a waveguide at a temperature of at most 250° C. to H2 (partial pressure greater than 1 atmosphere (14.7 psi or 9.65×104 Pa), such that irradiation can result in a normalized index change of at least 10−5. U.S. Pat. No. 5,500,031, a continuation-in-part of the above-mentioned '659 patent, speaks of a method of exposing the glass to hydrogen or deuterium at a pressure in the range of 14 psi (9.65×104 Pa) Pa to 11,000 psi (7.58×107 Pa) and at a temperature in the range 21° C. to 150° C. The parameters described in these references are probably typical for hydrogen-loading an optical fiber
The '031, '659 and '427 references point out problems with hydrogen loading methods in which temperatures exceed 250° C., or even 150° C. In teaching away from higher temperatures, the '659 Patent indicates that at high-temperatures “typical polymer fiber coatings would be destroyed or severely damaged” (column 1, lines 51-54). It further emphasizes the fact that “the prior art high temperature sensitization treatment frequently increases the optical loss in the fiber and/or may weaken the fiber” (column 1, lines 54-56). Finally, the '659 patent differentiates itself from the prior art by stating that a high temperature treatment involves “a different physical mechanism” than does a low-temperature treatment. For example, U.S. Pat. No. 5,235,659 explicitly indicates that temperatures of “at most 250° C.” should be used.
It has been observed that at higher temperatures the polymer coating, (usually an acrylate material), that protects the glass from harmful chemical reactions in a normal environment will degrade or oxidize (burn). Coatings that have degraded or oxidized and lost their protective value need to be removed and replaced, which can be a difficult and expensive process. Uncoated fiber is fragile, and requires great care during handling.
Most of the gratings written today by industry involve about 5 cm (2 inches or less) of the length of a fiber, depending on the type of grating to be written. Traditionally, it has been taught to place an entire length of optical fiber in a vessel containing hydrogen or deuterium atmospheres at certain temperatures and pressures. The grating manufacturing process usually entails a first process of placing a fiber spool in a hydrogen or deuterium containing vessel, placing the vessel in an oven and loading the entire fiber through the polymer coating.
To achieve the desired level of hydrogen in fiber with conventional hydrogenating methods (about 1 ppm), one will typically expose fiber to a hydrogen atmosphere for several days and, in some cases, for several weeks. Exemplary exposures such as 600 hours (25 days), 21° C., at 738 atm (10,800 psi or 7.48×107 Pa) or 13 days, 21° C. at 208 atm (3060 psi or 2.11×107 Pa) are reported as typical. Obviously, such long exposures extend the time required to fabricate optical devices that rely on photosensitive glass. Because of the long duration needed for traditional fiber hydrogenation, several pressure vessels are needed in a high-volume production environment to increase throughput and avoid idle time. These vessels are costly to install safely and increase the potential for serious accidents, especially when multiple vessels with separate control valves and gas supply cylinders are involved. Although installing multiple vessels can increase production throughput, the hydrogenation process hampers grating fabrication cycle time, thus new product and specialty product development time can be compromised severely.
Once the length of fiber has been hydrogen-loaded, the coating is stripped (mechanically, chemically or by other means) from the area where the grating is to be written. A technician then uses a source of actinic radiation to write each grating individually. The fibers are then annealed by again heating the fiber to reduce the degradation curve of the gratings. The portion of the fiber that was stripped is then recoated.
The traditional Bragg grating manufacturing processes are slow and do not lend themselves to mass manufacturing. The traditional hydrogen loading techniques require that the entire length of fiber be subject to the hydrogen loading and heating cycles. The need to expose the entire fiber may result in optical effects on the fiber and places constraints on materials, such as fiber coatings, that may be used. One negative effect of hydrogen loading at higher temperatures is that it may increase the optical loss characteristics of an optical fiber. Furthermore, high-temperature heating cycles may deteriorate optical fiber coatings.
It would be desirable to have the ability to provide an optical fiber which would allow the photosensitive writing of gratings, while reducing the deleterious effects of loading the entire fiber.